Circuit and method for reading and writing data in a memory device

ABSTRACT

A memory device. The memory device includes an array of memory cells that are coupled to a number of word lines and a number of digit lines. The memory device further includes an addressing circuit that is coupled to the array. The addressing circuit selects a memory cell based on a received address signal. An input/output device is coupled to the digit lines of the array. The input/output device includes an input for receiving a control signal. A control circuit is coupled to the input of the input/output device. The control circuit produces a control signal with a first voltage level when reading data from the array and produces a control signal with a second voltage level when writing data to the array. Thus, the control signal causes the input/output device to provide acceptable drive current during a read operation such that the input/output device does not disturb the data on the digit lines. Further, the control signal provides sufficient drive current during a write operation to allow the data on input/output lines to be passed to the digit lines.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to the field of electroniccircuits and, in particular, to a circuit and method for reading andwriting data in a memory device.

BACKGROUND OF THE INVENTION

Electronic systems typically store data during operation in a memorydevice. In recent years, the dynamic random access memory (DRAM) hasbecome a popular data storage device for such systems. Basically, a DRAMis an integrated circuit that stores data in binary form (e.g., "1" or"0") in a large number of cells. The data is stored in a cell as acharge on a capacitor located within the cell. Typically, the cells of aDRAM are arranged in an array so that individual cells can be addressedand accessed. The array can be thought of as rows and columns of cells.Each row includes a word line that interconnects all of the cells on therow with a common control signal. Similarly, each column includes adigit line that is coupled to at most one cell in each row. Thus, theword and digit lines can be controlled so as to individually access eachcell of the array.

To read data out of a cell, the capacitor of a cell is accessed byselecting the word line associated with the cell. A complimentary digitline that is paired with the digit line for the selected cell isequilibrated with the voltage on the digit line for the selected cell.When the word line is activated for the selected cell, the capacitor ofthe selected cell discharges the stored voltage onto the digit line,thus changing the voltage on the digit line. A sense amplifier detectsand amplifies the difference in voltage on the pair of digit lines. Aninput/output device for the array, typically an n-channel transistor,passes the voltage on the digit line for the selected cell to aninput/output line for communication to, for example, a processor of acomputer or other electronic system associated with the DRAM. In a writeoperation, data is passed from the input/output lines to the digit linesby the input/output device of the array for storage on the capacitor inthe selected cell.

One problem with DRAM design relates to sizing of the input/outputdevices of the memory array. Typically, the input/output devices aren-channel transistors that are two to eight times smaller than thetransistors in the sense amplifier. The ratio of transistor sizes usedin a specific design results from trade-offs that relate to the twodistinct operations of the input/output device, namely reading andwriting data. During a read operation, the input/output device shouldnot affect the voltage on the digit lines. If the input/output devicesare too big (e.g., provide too small of a resistance between the senseamplifier and the input/output lines), the input/output devices cantrigger the parasitic capacitance of the input/output lines orimbalances in the layout of the sense amplifier such that the data onthe digit lines is corrupted. Conversely, during a write operation, theinput/output devices need to be able to trigger the sense amplifier tomove the voltage on the digit lines to the power supply voltage andground potential. If the input/output devices of the array are toosmall, the devices will not provide sufficient current for triggeringthe sense amplifier when data is to be written to a selected cell overthe digit line. Thus, conventional designs require a trade-off withrespect to sizing the input/output devices of the memory array.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran improved circuit and method for reading and writing data in an arrayof a memory device.

SUMMARY OF THE INVENTION

The above mentioned problems with memory devices and other problems areaddressed by the present invention and which will be understood byreading and studying the following specification. A memory device isdescribed which uses an input/output device for the array that operateswith different drive levels for different access states. For purposes ofthis specification, one access state is the state of reading data from amemory cell. Another state is the state of writing data to a memorycell.

In particular, an illustrative embodiment of the present inventionincludes a method for writing and reading data in a memory device.According to this embodiment the method applies a control signal with afirst voltage level to an input/output device of the memory array whenreading data from the memory device. The method further applies acontrol signal with a second voltage level, different from the firstvoltage level, to the input/output device of the memory array whenwriting data in the memory device. Specifically, in one embodiment, thevoltage level of the control signal is higher during a write operation.By using different voltages on the control signal during differentstates of an access, the control signal causes the input/output deviceto provide acceptable drive current during a read operation such thatthe input/output device does not disturb the data on a pair of digitlines for the cell. Further, the control signal provides sufficientdrive current during a write operation to allow the data on input/outputlines to be passed to the digit lines.

In another embodiment, the present invention provides a method ofwriting and reading data in a memory device. According to thisembodiment, the method selects a cell of an array of the memory deviceto be accessed and charges a word line for the selected cell. The methodfurther activates a sense amplifier associated with the charged wordline. The method selectively drives an input/output device of the arraywith different current levels during different states of an accessoperation. In one embodiment, the method selectively drives an n-channeltransistor with different voltage levels during read and writeoperations.

In another embodiment, the present invention provides a memory device.The memory device includes an array of memory cells that are coupled toa number of word lines and a number of digit lines. The memory devicefurther includes an addressing circuit that is coupled to the array. Theaddressing circuit selects a memory cell based on a received addresssignal. An input/output device is coupled to the digit lines of thearray. The input/output device includes an input for receiving a controlsignal. A control circuit is coupled to the input of the input/outputdevice. The control circuit produces a control signal with a firstvoltage level when reading data from the array and produces a controlsignal with a second voltage level when writing data to the array.

In another embodiment, the present invention provides a method ofaccessing a selected cell of a memory device. The method determines theaddress of the selected cell and activates a sense amplifier that iscoupled to control the voltage on a pair of digit lines for the selectedcell of the memory device. The method further determines the type ofaccess, e.g., read or write, to be executed for the selected cell of thememory device. Based on the determination, the method generates avariable voltage control signal to activate input/output transistors ofthe memory array so as to couple data between the digit lines and a pairof complementary input/output lines.

In another embodiment, the present invention provides a memory device.The memory device includes an array of addressable memory cells that arecoupled to digit and word lines. An address circuit selects a cell inthe array. The memory device also includes a sense amplifier that iscoupled to control the voltage on a pair of digit lines for the selectedcell of the memory device. The memory device includes input/outputtransistors that couple the pair of digit lines to a pair ofinput/output lines. A control circuit of the memory device determinesthe type of access to be executed for the selected cell of the memorydevice and generates a variable voltage control signal to activate theinput/output transistors of the memory array based on the type of accessso as to couple data between the digit lines and the input/output lines.

In another embodiment, the present invention provides a memory devicefor storing data. The memory device includes a control circuit thatproduces signals to control the operation of the memory device. An arrayof cells of the memory device are accessed by a number of word and digitlines. The memory device also includes a number of input/output lines.Further, a number of input/output devices are driven by a variablevoltage control signal from the control circuit to couple data betweenthe input/output lines and a selected cell via the digit lines dependingon a state of the access.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an illustrative embodiment of the presentinvention;

FIG. 2 is a schematic diagram of an embodiment of a sense amplifiercircuit for use in a memory device according to the teachings of thepresent invention; and

FIGS. 3A through 3I are timing diagrams that illustrate signals for usewith, for example, the embodiments of FIGS. 1 and 2 according to theteachings of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration specific illustrative embodiments inwhich the invention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

FIG. 1 is a block diagram of an illustrative embodiment of the presentinvention. This embodiment includes memory device 100 that is coupled toelectronic system 102 by control lines 104, address lines 106 andinput/output (I/O) lines 108. Electronic system 102 comprises, forexample, a processor based computer, or other appropriate system forreading and writing data in a memory device. Memory device 100 includesarray of memory cells 110 that is coupled to word line decoder 114 andsense amplifier 111. Word line decoder 114 includes word line driversthat are coupled to word lines of array 110. Sense amplifier 111 iscoupled to digit line decoder 112. Digit line decoder 112 and word linedecoder 114 are coupled to address lines 106. Digit line decoder 112 iscoupled to I/O circuit 116. I/O circuit 116 is coupled to I/O lines 108.Control circuit 118 is coupled to control lines 104, sense amplifier111, word line decoder 114, digit line decoder 112, and I/O circuit 116.

In operation, electronic system 102 provides signals on address lines106 and control lines 104 when data is to be read from or written to acell of array 110. Word line decoder 114 determines the word line of aselected cell of array 110 using the address provided on address lines106. Further, digit line decoder 112 determines the digit line of theselected cell of array 110. In a read operation, sense amplifier 112detects the value stored in the selected cell based on word lines ofarray 110. Sense amplifier provides this voltage to I/O circuit 116which, in turn, passes data to electronic system 102 over I/O lines 108.In a write operation, I/O circuit 116 passes data from I/O lines 108 tosense amplifier 112 for storage in the selected cell of array 110.

Control circuit 118 of this embodiment advantageously uses differentvoltage levels for control signals provided to column input/output (I/O)devices of sense amplifier 112 during read and write operations.Specifically, during a read operation, control circuit 118 provides acontrol signal with a first voltage level to the column I/O devices ofsense amplifier 112. During a write operation, control circuit 118 usesa second, higher voltage level for the control signal to column I/Odevices of sense amplifier 112. For example, control circuit 118 couldprovide a voltage on the order of the power supply as a control signalduring a read operation and a pumped voltage (e.g., above the powersupply voltage) during a write operation.

FIG. 2 is a schematic diagram of an embodiment of a sense amplifier,indicated generally at 111a, constructed according to the teachings ofthe present invention. Sense amplifier 111a includes first and secondp-channel transistors, 201 and 203, respectively. Transistors 201 and203 are coupled between digit (D) and digit complement (D) lines 200 and202, respectively, in a cross-coupled configuration to form a p-senseamplifier. The p-sense amplifier is controlled by a signal, labeled ACT,that is provided to the gate of transistors 201 and 203. Digit and digitcomplement lines 200 and 202 are coupled to, for example, array 110 ofFIG. 1. Further, sense amplifier 111a includes isolation transistors 204and 206 that are coupled to digit and digit complement lines 200 and202, respectively. Isolation transistors 204 and 206 are also coupled tofirst and second n-type transistors 207 and 209 in a cross-coupledconfiguration as shown in FIG. 1. Transistors 207 and 209 form ann-sense amplifier that works in cooperation with the p-sense amplifierto drive the voltage on digit and digit complement lines 200 and 202 tothe power supply voltage and ground. Sense amplifier 111a also includesfirst and second input/output (I/O) transistors 210 and 212 that coupledigit and digit complement lines 200 and 202 to I/O and I/O complementlines 214 and 216, respectively. Transistors 210 and 212 are commonlyreferred to as column I/O transistors or devices

Sense amplifier 111a receives control signals from, for example, controlcircuit 118 and digit line decoder 112 of FIG. 1. A signal, labeled ISO,is coupled to the gates of isolation transistors 204 and 206.Additionally, a gate of each of I/O transistors 210 and 212 is coupledto receive a control signal, labeled CSEL. A latch signal, labeledNLAT*, is coupled to a gate of transistors 207 and 209.

In operation, sense amplifier 111a passes data between, for example,electronic system 102 and array of memory cells 110 of FIG. 1. Theoperation of sense amplifier 111a is described using the example ofreading a high logic level from digit line 200 and then writing a lowlogic level to the selected cell on digit line 200. FIGS. 3A through 3Iprovide exemplary timing diagrams that illustrate voltage levels ofvarious signals of sense amplifier 111a of FIG. 2 during this operation.

At time t₁, a control signal labeled RAS (Row Address Strobe)transitions from a high voltage level to a low voltage level. Thisindicates that an address of a row (e.g., word line) in array 110 for aselected cell is available on address lines 106. Word line decoder 114determines the word line for the selected cell of array 110. At time t₂,the word line for the selected cell is raised to a high voltage level(typically, a voltage level above the power supply voltage). With theword line at a high voltage level, the selected cell dumps charge ontodigit line 200 as shown after time t₂ in FIG. 3F. At this point thecontrol signal ISO is at a high voltage level such that the voltage ondigit line 200 is passed to transistors 207 and 209 of the n-senseamplifier.

At time t₃, NLAT* is brought to a low voltage level. This activates then-sense amplifier to drive the low potential digit line to ground. Inthis case, FIG. 3G shows that the voltage on digit complement line 202is driven to ground by transistors 207 and 209. At time t₄, the ACTsignal is brought to a high potential so as to activate transistors 201and 203 of the p-sense amplifier to drive the higher potential digitline (digit line 200) to, for example, approximately the value of thepower supply voltage as shown in FIG. 3F.

At time t₅, the CSEL signal transitions to a voltage level, V₁. Thisvoltage level provides acceptable drive current for I/O transistors 210and 212 to cause a separation in the voltage level on I/O and I/Ocomplement lines 214 and 216 that indicates to electronic system 102that a high logic value is stored in the selected cell. At time t₆, alow logic value is placed on I/O line 214 for storage in the selectedcell. At t₆, the CSEL signal is raised to a higher voltage level, V₂, sothat I/O transistors 210 and 212 provide higher drive current so as tocause sense amplifier 111a to drive digit and digit complement lines 200and 202 to appropriate levels to store the data provided on I/O and I/Ocomplement lines 214 and 216.

Advantageously, different voltage levels are used to drive I/O and I/Ocomplement transistors 210 and 212 during the read and write operationsto reduce negative effects of I/O line capacitance and parasiticimbalances of sense amplifier 208 during read operations. During a readoperation, a lower voltage, V₁, is used. This produces a reduced drivecurrent (alternatively, increases the effective resistance of thetransistor). The increased resistance alters the time constant for theparasitic capacitances so as to assure that the sense amplifier hassufficient time to drive the digit and digit complement lines to thecorrect values before the I/O lines can interfere. A higher voltage isused during a write operation because the goal is to have the voltage onthe I/O lines affect the voltage of the sense amplifier. The higher gatevoltage increases the drive current (reduces the resistance) so that thesense amplifier responds to the value of the voltage on the I/O and I/Ocomplement lines. This also allows a higher voltage (e.g., a voltageabove the power supply voltage) to be passed to the sense amplifier bytransistors 210 and 212.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, the type of sense amplifier used with the memorydevice can be altered without departing from the spirit and scope of thepresent invention. Further, the order and timing of the various controlsignals used during the read and write operations can be varied fromwhat is shown in FIGS. 3A through 3I.

What is claimed is:
 1. A method for writing and reading data in a memorydevice, the method comprising the steps of:applying a control signalwith a first voltage level to an input/output device of the memory arraywhen reading data from the memory device; and applying a control signalwith a second voltage level, different from the first voltage level, tothe input/output device of the memory array when writing data in thememory device.
 2. The method of claim 1, wherein the step of applying acontrol signal with a first voltage level comprises the step of applyinga control signal with a voltage level that is lower than the secondvoltage level.
 3. A method of writing and reading data in a memorydevice, the method comprising the steps of:selecting a cell of an arrayof the memory device to be accessed; charging a word line for theselected cell; activating a sense amplifier associated with the chargedword line; and selectively driving an input/output device of the arraywith different current levels during different states of an accessoperation.
 4. The method of claim 3, wherein the step of selectivelydriving an input/output device with different current levels comprisesthe step of selectively driving an input/output device with differentcurrent levels during read and write operations.
 5. The method of claim3, wherein the step of selectively driving an input/output devicecomprises the step of driving an n-channel transistor with differentvoltage levels during read and write operations.
 6. The method of claim3, wherein the step of selectively driving the input/output devicecomprises the steps of:driving the input/output device with a firstvoltage level during a read operation; and driving the input/outputdevice with a second, higher voltage level during a write operation. 7.A memory device, comprising:an array of memory cells that are coupled toa number of word lines and a number of digit lines; an addressingcircuit, coupled to the array, that selects a memory cell based on areceived address signal; an input/output device coupled to the digitlines of the array and having an input for receiving a control signal;and a control circuit, coupled to the input of the input/output device,that produces a control signal with a first voltage level when readingdata from the array and that produces a control signal with a secondvoltage level when writing data to the array.
 8. The memory device ofclaim 7, wherein the control circuit produces a control signal with avoltage level during a read operation that is lower than the voltagelevel of the control signal during a write operation.
 9. The memorydevice of claim 7, wherein the input/output device comprises ann-channel transistor.
 10. A system, comprising:a processor circuit; amemory device coupled to the processor circuit for writing and readingdata, the memory device including:an array of cells that are accessed bya number of word and digit lines based on signals from the processor; anumber of input/output lines that are communicatively coupled to theprocessor; and a number of input/output devices that are driven by avariable voltage control signal to couple data between the input/outputlines and the digit lines depending on a state of the access.
 11. Thesystem of claim 10, wherein the memory device includes a control circuitthat generates the variable voltage control signal with a first voltagelevel during a read state of the access and a second, different voltagelevel during a write state of the access.
 12. The system of claim 10,wherein the memory device includes a control circuit that adjusts thevariable voltage control signal with a first voltage level during a readoperation and a second, higher voltage level during a write operation.13. A method of accessing a selected cell of a memory device, the methodcomprising the steps of:determining the address of the selected cell;activating a sense amplifier that is coupled to control the voltage on apair of digit lines for the selected cell of the memory device;determining a type of access to be executed for the selected cell of thememory device; and generating a variable voltage control signal toactivate input/output transistors of the memory array based on the typeof access so as to couple data between the digit lines and a pair ofcomplementary input/output lines.
 14. The method of claim 13, whereinthe step of generating a variable voltage control signal comprises thesteps of:generating a control signal with a first voltage level during aread portion of an access; and generating a control signal with a secondvoltage level during a write portion of an access.
 15. The method ofclaim 13, wherein the step of generating a variable voltage controlsignal comprises the steps of:generating a control signal with a firstvoltage level during a read portion of an access; and generating acontrol signal with a second, higher voltage level during a writeportion of an access.
 16. A memory device, comprising:an array ofaddressable memory cells that are coupled to digit and word lines; anaddress circuit that selects a cell in the array; a sense amplifier thatis coupled to control the voltage on a pair of digit lines for theselected cell of the memory device; input/output transistors that couplethe pair of digit lines to a pair of input/output lines; and a controlcircuit that determines a type of access to be executed for the selectedcell of the memory device and that generates a variable voltage controlsignal to activate the input/output transistors of the memory arraybased on the type of access so as to couple data between the digit linesand the input/output lines.
 17. The memory device of claim 16, whereinthe input/output transistors comprise n-channel transistors.
 18. Thememory device of claim 16, wherein the sense amplifier comprises an pairof cross-coupled n-channel transistors.
 19. A memory device for writingand reading data, the memory device comprising:a control circuit thatproduces signals to control the operation of the memory device; an arrayof cells that are accessed by a number of word and digit lines; a numberof input/output lines; and a number of input/output devices that aredriven by a variable voltage control signal from the control circuit tocouple data between the input/output lines and a selected cell via thedigit lines depending on a state of the access.
 20. The memory array ofclaim 19, wherein the control circuit produces the variable controlsignal with a first voltage level during a read operation and with adifferent, higher voltage level during a write operation.